Solid State Chemistry 1 States of matter The

Solid State Chemistry

1 - States of matter The matter has three basic states, Solid, Liquid and Gas (Plasma or ionized gas is considered as a fourth state of matter. ) Ø Gases and liquids can flow and take up the shape of their container. Solids, on the other hand, are rigid and have a definite volume and shape. Ø In both gases and liquids, atoms, ions and molecules translate randomly as well as rotate and vibrate which explains the ability of gases and liquids to flow. In solids, atoms, ions and molecules are held together by relatively strong chemical forces. So that, They do not translate or rotate and can only oscillate about their mean positions (vibration) which explains why solids are rigid and have definite shape.

Ø Solid State is generally characterized by rigidity, mechanical strength, and incompressibility due to close packing of constituent particles of matter (atoms, molecules or ions). The atoms, molecules or ions are held together by strong forces which adhere them to occupy fixed positions (called lattice points) in a three-dimensional structure called the solid lattice. 2 - Types of solids Broadly speaking, solids are of two types : (a) Crystalline solids; also called true solids (b) Amorphous solids (amorphous = no form) A crystalline solid exists as small crystals, each crystal having a characteristic geometrical shape. In a crystal, the atoms, molecules or ions are arranged in a regular, repeating three dimensional pattern called the crystal lattice. An amorphous solid has atoms, molecules or ions arranged in a random way without ordered crystalline lattice.

Examples Sugar crystals (Crystalline) Sugar cotton candy (Amorphous)

3 - Isotropy and Anisotropy of solids Isotropic material is that material in which the physical and mechanical properties are the same in all directions and anisotropic material is that material in which the physical and mechanical properties differ with orientation Amorphous substances are said to be isotropic because they exhibit the same value of any property (e. g. Refractive index, Thermal and electrical conductivities, Coefficient of thermal expansion, etc) in all directions. This is due to the random arrangement of particles in the amorphous substances (as in liquids) which make all the directions to be equivalent and properties are independent of direction. In the figure, When a property is measured along A and B directions, the value along A direction will be similar to that along B direction. This is so because there are two types of particles along the two directions.

Crystalline substances, on the other hand, are anisotropic and the magnitude of a physical property varies with directions. For example, in a crystal of silver iodide, the coefficient of thermal expansion is positive in one direction and negative in the other. Similarly, velocity of light in a crystal may vary with direction in which it is measured. In the figure, When a property is measured along A and B axes, the value along A axis will be different from that along B axis. This is so because there is only one type of particles along A axis while there are two types of particles in alternate positions along B axis.

4 - The habit of crystal and the interfacial angles The external shape is called the habit of the crystal, the plane surfaces of the crystal are called faces and the angles between the faces are referred to as the interfacial angles. The external appearance or size of crystals (Habit of the crystal) of a substance may be different depending on the conditions of crystal growth. For example, slow growth from a slightly super-saturated solution or a very slowly cooling solution gives large crystals. In addition, in the presence of certain impurities, different faces grow at different rates and give rise to many forms. However, the interfacial angles are always the same for a certain substance. Interfacial angle

5 - Symmetry of crystals In addition to the angles, another important property of crystals is their symmetry. Symmetry in crystals may be due to a plane, a line or a point. Accordingly there are three types of symmetry elements associated with a crystal. These are called the Elements of Symmetry 1 - Plane of Symmetry A crystal is said to have a plane of symmetry if it can be divided by an imaginary plane into two equal parts, each of which is the exact mirror image of the other.

2 - Axis of Symmetry An axis of symmetry is an imaginary line drawn through the crystal such that during rotation of the crystal through 360°, the crystal presents exactly the same appearance more than once. If similar view appears twice, it is called an axis of two fold symmetry or diad axis. If it appears thrice, it is an axis of three fold symmetry or triad axis, and so on.

3 - Centre of Symmetry (Point of symmetry) It is a point at the centre of the crystal so that any line drawn through it will meet the surface of the crystal at equal distances on either side. The crystal may have a number of planes of symmetry or axes of symmetry but it can have only one centre of symmetry